Mk.5 / W5
The Mark 5 was a smaller and lighter implosion weapon than previous designs. The Mark 5 was the first lightweight strategic nuclear bomb. Its weight was one-half that of the Nagasaki weapon. It was an improved version of the "Fat Man" bomb that was dropped on Nagasaki by a B-29 during World War II. This high efficiency implosion bomb could be configured to yield three times the explosive power of the Fat Man. The Mark 5's nuclear material was normally kept separate from the casing and inserted just before it was dropped. The nuclear warhead was loaded through the doors in the casing.
The experience gained in the successful development of the Mark 4 put Los Alamos Laboratory in a position to move much more rapidly and with more assurance on the development of other new systems. A smaller and lighter weapon, called the Mark 5, was tested successfully in 1951. First deployed in 1952 and remained in the inventory until 1963. The Mark 5 could be carried by the B-29, B-36, B-45, B-47, and the B-50. It weighted about 3,175 pounds with an explosive yield in the kiloton range (1 kiloton is equivalent to 1,000 tons of TNT).
The Mark 5 nuclear bomb and the W5 warhead had the same nuclear explosive device developed in the early 1950s, and was in service from 1952 to 1963. Mark 5 was the first serial American nuclear device having a diameter of an implosive device of only 99.1 cm - much less than the first American nuclear bomb Fat Man. The Mark had a 92-point implosion system and a composite uranium-plutonium core.
The Mark 5 and W5 nuclear explosive had a diameter of 99.1 cm and a length of 193 cm. The diameter of the Mark 5 bomb as a whole is 111.8 cm, and the length is from 327.7 to 335.3 cm. Different variants of Mark 5 weigh from 1372.1 to 1440.2 kg. The weight of the warhead W5 is from 1090.9 to 1202 kg.
At least four basic core designs were created, as well as their modifications with a yield of 6, 16, 55, 60, 100 and 120 kt. Like many other early US nuclear devices, fissile material or pit should be stored separately from the bomb and inserted into it immediately before departure. This technology is known as In Flight Insertion or IFI. Mark 5 had an automatic IFI mechanism that could insert a pit into the assembly center from the storage position in the bow of the bomb.
In contrast with the Mk 6, which requires manual insertion of the nuclear material, the Mk 5 has a built in mechanism to perform the insertion of the capsule and the cored high explosives. Thus, by a switch on his infilght control box, the bomb commander of the delivery aircraft can automatically perform a nuclear insertion or extraction at any time. After the cartridge has been checked and installed, the tail section is attached, completing the assembly of this internally carried weapon.
The Mk5 was a large open pit weapon containing a large amount of high explosives. The front end contained the automatic in-flight insertion mechanism (AIFI), eight MC-300 impact fuses and two Horn Antennae. The fusing and firing set, collectively known as the cartridge, was attached to the HE sphere at the aft end. Two power supply assemblies were attached to the cartridge. The arm/safe switch, also a part of the cartridge, could be monitored and set from an access door in the tail assembly. The ballistic case followed aircraft construction, the aluminum skin was riveted to support rings. The case was in three sections, the front, the center (made up of three panels), and the tail assembly.
A total of 140 Mark-5 bombs were produced.
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